The present invention pertains to a drive signal supply circuit for a switching regulator. More specifically, the present invention pertains to a drive signal supply circuit for a switching regulator characterized by the fact that it produces a stable output voltage even for deviations of the power source voltage.
Usually, a regulator is used as a device for supplying a stable DC voltage to a load.
An example of a conventional switching regulator is the switching regulator 101 shown in FIG. 13. Switching regulator 101 comprises switching transistor 102, rectifying/smoothing circuit 180, and controller 127 to be explained later.
Switching transistor 102 is an n-channel MOS transistor (hereinafter referred to as nMOSFET). Its drain terminal is connected to a power source voltage supply line that supplies power source voltage Vcc. Its gate terminal is connected to controller 127 to be explained later, and with this constitution switching transistor 102 can be turned ON/OFF corresponding to the output signal of controller 127.
Rectifying/smoothing circuit 180 comprises rectifying MOSFET 103, choke coil 105, and smoothing capacitor 106. The source terminal of switching transistor 102 is connected to one end of choke coil 105. The other end of choke coil 105 is connected to one end of smoothing capacitor 106, and, at the same time, this end is connected through output terminal 107 to one end of load 108. The other end of the load is grounded. The other end of smoothing capacitor 106 is grounded.
Rectifying MOSFET 103 is formed by an nMOSFET. Its drain terminal is connected to the source terminal of switching transistor 102, its source terminal is grounded, and its gate terminal is connected to controller 127. Its constitution is dependent on the output signal of controller 127, such that it is OFF when switching transistor 102 is ON, and it is ON when switching transistor 102 is OFF. Also, in the figure, 104 represents the internal parasitic diode of rectifying MOSFET 103.
In said switching regulator 101, when rectifying MOSFET 103 is OFF, switching transistor 102 is turned from OFF to ON. As a result, choke coil 105 is connected through switching transistor 102 to the power source voltage supply line, current flows in choke coil 105, and this current also flows through output terminal 107 into load 108.
In this state, if switching transistor 102 is turned OFF and rectifying MOSFET 103 is turned ON, an electromotive force is generated between the two terminals of choke coil 105. Due to this electromotive force, a negative voltage is asserted on the drain terminal of rectifying MOSFET 103. As a result, parasitic diode 104 inside rectifying MOSFET 103 is forward-biased, and the energy stored in choke coil 105 is supplied to load 108.
The ON/OFF state of said switching transistor 102 is controlled by the voltage output from driver 125 of controller 127 to be explained later, and switching transistor 102 is turned ON/OFF repeatedly. Here, the potential of output terminal 107 varies correspondingly. However, because smoothing capacitor 106 is connected in parallel to load 108, in company with the ON/OFF switching, there is repeated charging/discharging of said smoothing capacitor 106, so that the potential at output terminal 107 is smoothed. The smoothed voltage is output from output terminal 107 as an output voltage across load 108. While this output voltage is asserted on load 108, it is also input to controller 127.
Controller 127 comprises voltage dividing circuit 122, reference voltage generating source 119, error amplifier 111, comparator 112, sawtooth wave generating circuit 113, driver 125, and negative feedback circuit 128. Voltage dividing circuit 122 is composed of two resistors 1211 and 1212 which are connected in series between output terminal 107 and ground. The output voltage is input to voltage dividing circuit 122, and the output voltage is divided according to the resistance ratio of resistors 1211 and 1212 to generate a sampling voltage which is input to the inverting input terminal of error amplifier 111. Reference voltage generating source 119 is connected to the non-inverting input terminal of error amplifier 111, so that reference voltage Vref is input from reference voltage generating source 119. Between the output terminal of error amplifier 111 and the inverting input terminal, there is negative feedback circuit 128 formed by a series circuit of a resistor and a capacitor. From error amplifier 111, the voltage of the error difference between reference voltage Vref and the sampling voltage is amplified by a prescribed gain determined by the impedance of negative feedback circuit 128, and it is then output.
The output voltage of error amplifier 111 is input to the non-inverting input terminal of comparator 112. A sawtooth wave is input from sawtooth wave generating circuit 113 to the inverting output terminal of comparator 112. Comparator 112 compares the output voltage of error amplifier 111 and the sawtooth wave, and it outputs a pulse signal which defines the ON period of switching transistor 102.
Said driver 125 turns switching transistor 102 ON/OFF corresponding to the pulse signal. When the output voltage rises higher than a prescribed voltage value, the output is lowered. On the other hand, when the output voltage falls lower than a prescribed voltage value, the output voltage is raised by means of driver operation. By means of this operation, it is possible to keep the output voltage at a prescribed level.
Because the response speed of error amplifier 111 in said conventional switching regulator 101 is slow, when there are rapid fluctuations in power source voltage Vcc, it becomes impossible for error amplifier 111 to respond to these fluctuations, and the output voltage becomes unstable.
In FIG. 14, curves (J)-(M) show the operation waveforms of the various circuits that form conventional switching regulator 101 when power source voltage Vcc is constant.
Curve (J) represents the waveform of the sawtooth wave output from sawtooth wave generating circuit 113; curve (K) represents the output waveform of error amplifier 111; curve (L) represents the output waveform of comparator 112; curve (M) represents the waveform of the source terminal of switching transistor 102.
FIG. 15 is a diagram illustrating the operations of the various circuits when the power source voltage Vcc falls rapidly. In FIG. 15, curve (N) represents the waveform of power source voltage Vcc that falls rapidly; curve (O) represents the waveform of the output voltage from comparator 112; curve (P) represents the waveform of the source terminal potential of switching transistor 102; curve (Q) represents the waveform of the sawtooth wave; curve (R) represents the output waveform of error amplifier 111; and curve (S) represents the waveform of the output voltage.
As can be seen, when the power source voltage Vcc falls rapidly according to curve (N), error amplifier 111 cannot respond to such a rapid change. Consequently, the output of error amplifier 111 becomes unstable, so that the output voltage also becomes unstable as shown in curve (S). It becomes stable again after the transition period T1 shown in FIG. 15.
FIG. 16 illustrates the operations of the various circuits when power source voltage Vcc rises rapidly. In FIG. 16, curve (T) represents the waveform of power source voltage Vcc that rises rapidly, and curve (U) represents the waveform of the output voltage of comparator 112. Curve (V) represents the waveform of the source terminal potential of switching transistor 102; curve (W) represents the waveform of the sawtooth wave; curve (X) represents the waveform of the output of error amplifier 111; and curve (Y) represents the waveform of the output voltage.
As can be seen, when power source voltage Vcc rises rapidly according to curve (T), error amplifier 111 cannot respond to the rapid change of the power source voltage. As a result, as can be seen in curve (Y), peak voltage Vpeak is generated on the output voltage.
Thus, since it is impossible to respond to the rapid change in power source voltage Vcc, the output voltage of the switching regulator becomes unstable. This is a problem.
The purpose of the present invention is to solve the aforementioned problems of the conventional methods by providing a drive signal supply circuit for a switching regulator characterized by the fact that it can provide a stable output voltage even for deviations in the power source voltage.
One aspect of the invention is a drive supplycircuit which which supplies a drive signal to a switching transistor of a switching regulator comprising the switching transistor, a coil, a smoothing capacitor, and a flywheel diode, characterized by the following parts: a detecting circuit which detects the output voltage of the switching regulator; an error amplifier which compares the detection voltage output from said detecting circuit and a reference voltage, and which generates an error signal; a sawtooth wave signal generating circuit which generates a sawtooth wave signal; an amplitude center adjusting circuit which accepts said sawtooth wave signal as an input, which changes the amplitude center voltage of said sawtooth wave signal corresponding to the power source voltage, and which outputs an adjusted sawtooth wave signal; a comparator which compares said error signal and said adjusted sawtooth wave signal, and which generates a pulse signal for controlling the ON period of said switching transistor; and a driving circuit which generates a drive signal based on said pulse signal output from said comparator and which supplies the drive signal to said switching transistor.
Another aspect of the invention is the drive signal supply circuit the amplitude of said adjusted sawtooth wave signal is constant.
A futher aspect of the invention is a drive signal supply circuit which supplies a drive signal to a switching transistor of a switching regulator comprising the switching transistor, a coil, a smoothing capacitor, and a flywheel diode, characterized by the following parts: a detecting circuit which detects the output voltage of the switching regulator; an error amplifier which compares the detection voltage output from said detecting circuit and a reference voltage, and which generates an error signal; a sawtooth wave signal generating circuit which generates a sawtooth wave signal; an amplitude adjusting circuit which accepts said sawtooth wave signal as an input, which changes the amplitude of said sawtooth wave signal corresponding to the power source voltage and which outputs an adjusted sawtooth wave signal; a comparator which compares said error signal and said adjusted sawtooth wave signal, and which generates a pulse signal for controlling the ON period of said switching transistor; and a driving circuit which generates a drive signal based on said pulse signal output from said comparator and which supplies the drive signal to said switching transistor.
A still further aspect of the invention is a drive signal supply circuit where the amplitude center voltage of said adjusted sawtooth wave signal is constant.
Yet another aspect of the invention is a drive signal supply circuit which supplies a drive signal to a switching transistor of a switching regulator comprising the switching transistor, a coil, a smoothing capacitor, and a flywheel diode, characterized by the following parts: a detecting circuit which detects the output voltage of the switching regulator; an error amplifier which compares the detection voltage output from said detecting circuit and a reference voltage, and which generates an error signal; a sawtooth wave signal generating circuit which generates a sawtooth wave signal; an adjusting circuit which accepts said sawtooth wave signal as an input, which changes the amplitude center voltage of said sawtooth wave signal and the amplitude of said sawtooth wave signal corresponding to the power source voltage, and which outputs an adjusted sawtooth wave signal; a comparator which compares said error signal and said adjusted sawtooth wave signal, and which generates a pulse signal for controlling the ON period of said switching transistor; and a driving circuit which generates a drive signal based on said pulse signal output from said comparator and which supplies the drive signal to said switching transistor.
According to one aspect of the invention, switching regulator of the present invention comprises an amplitude center adjusting circuit. It detects the power source voltage, and, corresponding to the magnitude of the power source voltage, it changes the amplitude center voltage of the sawtooth wave signal and outputs this adjusted signal to the comparator.
Consequently, even when the power source voltage makes a rapid change, the center voltage of the sawtooth wave signal is raised/lowered at a high speed matching the rapid change, and it is output to the comparator. Consequently, the pulse signal output from the comparator can follow the rapid change in the power source voltage. Consequently, the output voltage can follow the rapid change in the power source voltage. As a result, it is possible to solve the problem of the prior art where the output voltage becomes unstable because it cannot follow the changes in the power source voltage.
Also, the overall gain of the switching regulator is a product of the gains of the error amplifier, comparator, and output section. The gain of the comparator is equal to (power source voltage)/(amplitude of the sawtooth wave signal), and it is directly proportional to variations in the power source voltage.
Consequently, since the power source voltage fluctuates in a conventional switching regulator, the gain of the comparator varies correspondingly, so that the overall gain of the switching regulator varies, and the output voltage is prone to noise. This is a problem.
However, another switching regulator of the present invention has an amplitude adjusting circuit, and corresponding to the magnitude of the power source voltage, the amplitude of the sawtooth wave signal is changed and output to the comparator.
Consequently, when the power source voltage becomes larger, the amplitude of the sawtooth wave signal also becomes larger. When the power source voltage becomes smaller, the amplitude of the sawtooth wave signal also becomes smaller. Consequently, the gain of the comparator equal to the ratio of the power source voltage to the amplitude of the sawtooth wave signal remains almost constant and thus, it is nearly independent of fluctuations in the power source voltage.
Consequently, even when the power source voltage varies, the overall gain of the switching regulator remains nearly constant. Consequently, there will be little noise in the output voltage.